He plays in a Frank Zappa-influenced band called 'Sons of Mr. Greengenes'.

Has two sisters, Sile & Orla

First name pronounced "Kill-ian"

He was planning a career in law until he discovered the world of acting.

Although he is a vegetarian, he learned to chop up meat in an abattoir for his role as butcher in Girl with a Pearl Earring (2003).

Auditioned for the role of Batman in 'Batman Begins.' The role was eventually given to Christian Bale. But, director Christopher Nolan liked Cillian's audition so much that he gave him a role as Batman's arch-nemesis Jonathan Crane/Scarecrow, who is a developing character in this movie.